The present invention is directed to a method of discontinuous manufacture of multilayer, co-extruded, hose-like preforms from thermoplastics material for the production of large volume, multilayer hollow members in a divided blow mold, where at least two different annularly shaped material melts are brought together in series within annular flow chamber of an annular piston serving as a co-extrusion head. The flow channel forms a multilayer material melt and the channel widens in a funnel-like manner in the direction away from the inlet of the material melts. From the annular channel, the combined multilayer material melts enter an annular storage space and are subsequently ejected through a narrow annular gap by the annular piston moving in its axial direction. The piston is pressed upwardly by the inflowing multilayer material melt. The invention is also directed to an apparatus for carrying out the method.
A method and apparatus for performing the method are disclosed in DE-OS 36 35 334. In this apparatus, the individual extruders are connected in the radial direction to the coextrusion head or annular piston. Accordingly, the material melt supplied from an extruder discharges directly radially into the movable annular piston. The extruders are pivoted in case of axial displacement of the annular piston. This pivoting motion of the extruders results in cross-sectional changes in the transition region between the exit channel of the extruder and the entrance channel of the annular piston, and such cross-sectional changes have a deleterious effect on the hose-like preform produced. Due to this arrangement, the preform may have weak points in the region of the flow lines of the individual material melts, and, during the blowing operation of the preform, can result in considerable impairment of the layer thickness and possibly even in the destruction of a layer. Such a result renders the finished hollow member useless.